The fourth Gordon Research Conference on Motile and Contractile Systems will provide an informal forum for recent advances and new directions in this active field. The focus of this conference will be on the actin-based and microtubule-based cytoskeletal components, their structures and important domains, their regulators and their motors. These topics are relevant to many fields including musculoskeletal disease, respiratory disease, development, transformation, oncogenesis as well as the normal functioning of the organism. The conference will include approximately 30 formal presentations by ihvited speakers in 9 lecture-discussion sessions with a major emphasis on discussion by all participants. The other participants, including junior scientists in particular, will be encouraged to present posters of their work and 2 formal poster sessions will be scheduled. This basic format has worked well in the previous 3 conferences, and we plan to continue emphasizing a minimum of formal presentations in order to maximize discussion and exchange of new unpublished results and techniques. The conference will begin with a session (1) on structural studies on actin and actin binding proteins which is now providing new information about important domains at the atomic level. This will be followed by session (2) on the large family of actin binding proteins, their domains, biochemical properties and regulation and function in actin-based cytoskeletal assembly and disassembly. The next two sessions (3 & 4) will highlight the explosive new information coming forth on the myosin families of actin-based motors, their relationships, regulation and functions in different types of cell motility and morphogenesis. This leads into the session (5) on the dynamics of the cell surface, including the role of the membrane cytoskeleton in cell surface signal transduction, adhesion, protrusion and locomotion. The next session (6) focuses on the mechanism and regulation of microtubule assembly during the cell cycle, including contributions from tubulin binding proteins and nucleation centers. Sessions (7 & 8) concentrate on the rapidly advancing information on microtubule-based motors, like the dynein and kinesin families, their structural relationships, motility, in vivo functions and regulation. The last formal lecture-discussion session (9) introduces new developments on the function of cytoskeletal components in mitosis, cytokinesis, and development. There will also be two formal poster sessions, although participants will be encouraged to leave their posters up throughout the meeting to promote discussion. Genetic and molecular biological experimental methods for identifying new family mmbers and for analyzing function will be integrated into all the sessions.